Measurements by a laser altimeter instrument orbiting aboard
NASA's Mars Global Surveyor spacecraft are providing striking new
views of the north pole of the red planet and the processes that
have shaped it.

This first three-dimensional picture of Mars' north pole
enables scientists to estimate the volume of its water ice cap
with unprecedented precision, and to study its surface variations
and the heights of clouds in the region for the first time.

The elevation measurements were collected by the Mars
Orbiter Laser Altimeter aboard Global Surveyor during the spring
and summer of 1998, as the spacecraft orbited Mars in an interim
elliptical orbit. The altimeter sends laser pulses toward the
planet and measures the precise amount of time before the
reflected signals are received back at the instrument. From this
data, scientists can infer surface and cloud heights.

Approximately 2.6 million of these laser pulse measurements
were assembled into a topographic grid of the north pole with a
spatial resolution of 1 kilometer (six-tenths of a mile) and a
vertical accuracy of 5 to 30 meters (15 to 90 feet). A peer-
reviewed paper based on the measurements will be published in the
December 11 issue of Science magazine.

The topographic map reveals that the ice cap is about 1,200
kilometers (750 miles) across, with a maximum thickness of 3
kilometers (1.8 miles). The cap is cut by canyons and troughs
that plunge to as deep as 1 kilometer (0.6 mile) beneath the
surface. "Similar features do not occur on any glacial or polar
terrain on Earth," said Dr. Maria Zuber of the Massachusetts
Institute of Technology and NASA's Goddard Space Flight Center,
Greenbelt, MD. "They appear to be carved by wind and evaporation
of ice."

The altimeter data also reveal that large areas of the ice
cap are extremely smooth, with elevations that vary by only a few
feet over many miles. In some areas the ice cap is surrounded by
large mounds of ice, tens of miles across and up to half a mile
in height. "These structures appear to be remnants of the cap
from a time when it was larger than at present," Zuber said.
Impact craters surrounding the cap appear to be filled with ice
and dust that was either deposited by wind or condensation, or
perhaps remains from an earlier period when the ice cap was
larger.

The shape of the polar cap indicates that it is composed
primarily of water ice, with a volume of 1.2 million cubic
kilometers (300,000 cubic miles). The cap has an average
thickness of 1.03 kilometers (0.64 mile) and covers an area 1.5
times the size of Texas. For comparison, the volume of the
Martian north polar cap is less than half that of the Greenland
ice cap, and about four percent of Earth's Antarctic ice sheet.

The estimated volume of the north ice cap is about 10 times
less than the minimum volume of an ancient ocean that some
scientists believe once existed on Mars. If a large body of
water once existed on the red planet, the remainder of the water
must presently be stored below the surface and in the much
smaller south polar cap, or have been lost to space. But such a
large amount of unaccounted-for water is not easily explained by
current models of Martian evolution.

During its mapping of the north polar cap, the altimeter
instrument also made the first direct measurement of cloud
heights on the red planet. Reflections from the atmosphere were
obtained at altitudes from just above the surface to more than
about 15 kilometers (nine miles) on about 80 percent of the laser
profiles. Most clouds were observed at high latitudes, at the
boundary of the ice cap and surrounding terrain.

Clouds observed over the polar cap are likely composed of
carbon dioxide that condenses out of the atmosphere during
northern hemisphere winter. Many clouds exhibit dynamic
structure probably caused by winds interacting with surface
topography, much as occurs on Earth when winds collide with
mountains to produce turbulence.

The principal investigator for the instrument is Dr. David
E. Smith of the Goddard center. The instrument was designed and
built by the Laser Remote Sensing Branch of the Laboratory for
Terrestrial Physics at Goddard. The Mars Global Surveyor Mission
is managed by NASA's Jet Propulsion Laboratory, Pasadena, CA, for
the NASA Office of Space Science.

Further information about the altimeter and images created
from its measurements of the north pole are available on the
Internet at the following address: